The present invention relates to an amplitude limiting circuit and, more particularly, to an amplitude limiting circuit which can be suitably used to limit an input to a transmission power amplifier incorporated in a CDMA communication apparatus.
Recent digital mobile communication systems often use CDMA (Code Division Multiple Access) communication apparatuses to improve the interference resistance ability between communication channels.
In a CDMA communication apparatus, since the instantaneous power at the time of transmission is much higher than the average power, the linearity of a transmission power amplifier must be maintained up to a very high output level to suppress spreading of a transmission spectrum due to nonlinear distortion so as to reduce adjacent channel leakage power.
A power amplifier designed to have good linearity up to a very high amplitude has a large circuit size, and hence increases in cost and power consumption. For this reason, as a transmission power amplifier for a CDMA communication apparatus, a nonlinear compensation amplifier is used, which exhibits good linearity with respect to small amplitude components and nonlinearity with respect to large amplitude components. As shown in FIG. 5, the linearity of this nonlinear compensation amplifier has the input/output characteristics that linearity is maintained up to its maximum output, and when an input amplitude exceeds a value corresponding to the maximum output, the output level becomes constant.
In a transmission power amplifier having input/output characteristics like those shown in FIG. 5, when a signal corresponding to a level exceeding the maximum output is input, a transmission output is saturated to increase nonlinear distortion. As a consequence, the transmission spectrum spreads to increase adjacent channel leakage power, as indicated by the spectrum waveform without “CLIPPING” in FIG. 6.
It is therefore desirable to maximize the saturation power (maximum output) of the transmission power amplifier. In this case, however, as described above, the circuit size increases to increase power consumption and cost. For this reason, it is important in the CDMA communication apparatus that the instantaneous maximum power of an input signal is limited so as not to exceed the saturation power of the transmission power amplifier.
The simplest method of limiting the input amplitude of the transmission power amplifier is to clip an input signal with a predetermined value. If, however, an input signal is simply clipped, the nonlinear distortion of a signal waveform increases. As a result, the transmission spectrum further spreads, as indicated by the spectrum waveform obtained by “SIMPLE CLIPPING” in FIG. 6. As shown in FIG. 7, therefore, an amplitude limiting circuit is generally provided before a band limiting filter.
An amplitude limiting circuit 20 shown in FIG. 7 includes an amplitude converter 21 which calculates the amplitude value of an input signal constituted by an in-phase component I and quadrature component Q, a determination unit 22 which compares the amplitude value calculated by the amplitude converter 21 with a preset threshold to output a control value for limiting the amplitude of an input signal that exceeds the threshold, and a clipping circuit 23 which limits an output amplitude to a value equal to or less than the threshold in accordance with the control value output from the determination unit 22.
Of an output signal from the amplitude limiting circuit 20, only a predetermined baseband component is output from a filter 24. In this arrangement, since the frequency component of an output signal is limited by the filter 24 within a predetermined band, the transmission spectrum does not spread.
In the arrangement shown in FIG. 7, however, since the amplitude limiting circuit and filter 24 differ in their sampling rates for signal processing, level variations occur at the time of sampling. As a result, the amplitude of the signal passing through the filter 24 may increase again.
In addition, when the amplitude limiting circuit shown in FIG. 7 is applied to a CDMA communication apparatus of a so-called multicarrier amplification scheme of combining a plurality of carrier signals, shown in FIG. 8, the amplitude of the signal is increased again by processing after amplitude limitation.
The CDMA communication apparatus shown in FIG. 8 includes a plurality of amplitude limiting circuits 311 to 31N (N is a positive integer) which limit the amplitudes of input signals and a plurality of filters 321 to 32N which pass only predetermined band components.
The CDMA communication apparatus also includes first frequency converters 331 to 33N which convert input signals as baseband signals into signals having different frequencies for the respective channels, and a carrier combining unit 34 which combines output signals from the first frequency converters 331 to 33N.
The CDMA communication apparatus further includes a D/A converter 35 which converts the signal obtained by carrier combining into an analog signal, a second frequency converter 36 which converts the output signal from the D/A converter 35 into an RF signal, and a transmission power amplifier 37 which amplifies the RF signal to power necessary for transmission.
The CDMA communication apparatus includes the amplitude limiting circuits 311 to 31N, filters 321 to 32N, and first frequency converters 331 to 33N for the respective channels.
In this arrangement, since signals corresponding to a plurality of channels are subjected to vector combining in the carrier combining unit 34 after amplitude limitation, the effect of the amplitude limitation is lost. As a consequence, the transmission spectrum spreads, as indicated by the spectrum waveform of “CONVENTIONAL SCHEME” in FIG. 9.